The invention relates to a force limiter comprising a control member displaceable with respect to a holder and a chamber, whose shape is determined inter alia by the holder and the control member and which is located between a gas supply and a leakage resistance.
The invention also relates to a device for ultrasonically deforming an article made of synthetic material by means of a sonotrode that can be pressed against the article by a displacement device.
The invention further relates to a method of limiting a contact force with which a first article displaceable by a motor drive is pressed against a second article, the motor drive when switched into operation being switched off due to the contact force produced between the two articles getting into contact with each other.
In a so-called force-to-pressure converter known from German Patent Application DE-A1-3326138, an air current at a pressure which is kept constant exerts a force on a ball connected to the control member. A counter force exerted on the control member in the form of a rod ensures, when the force exerted by the air current is exceeded, a relative displacement between the rod (control member) and a housing (holder). The leakage control provided between the ball and an edge of an opening in the housing and located between an air inlet, a chamber and an air outlet connected thereto varies comparatively strongly upon a displacement of the rod with the ball. This involves the disadvantage that the force exerted on the ball by the air current can be kept constant only with great difficulty. Such a disadvantage is only recognized if the recognition is obtained that a substantially constant force is necessary if the known force-to-pressure converter should be used as a force limiter. This use has not been stated, however, in German Patent Application DE-A1-3326138.
Further, a device for ultrasonically deforming, for example, polymeric synthetic materials by means of a sonotrode that can be pressed by a displacement device against the synthetic material is known from the book "Ultrasonic Engineering" by J. R. Frederick (1965), pp. 191-192. The said book does not state the manner in which the contact force between the sonotrode and the synthetic material to be deformed is limited.
Finally, it should be noted that it is well known that, when in methods a first article is pressed against a second article, the motor drive is switched off at the instant at which contact is made between the two objects. Overshoot of the first article due to the mass inertia involves the risk of an inadmissibly great contact force between the articles.
The invention has for its object to provide a force limiter, in which the force (force of pre-stress) exerted by a gas (air) on the control member is substantially independent (constant) of relative displacements different in value between the holder and the control member.
The force limiter according to the invention is for this purpose characterized in that the leakage control comprises a gap which is connected to the chamber, is located between at least one circular-cylindrical inner surface and at least one circular-cylindrical outer surface and is annular in sectional views transverse to the direction of displacement of the control member.
A particular embodiment of the force limiter, which can be manufactured in a comparatively simple manner, is further characterized in that the chamber accommodates a circular-cylindrical sliding pin, which is slidable parallel to the direction of displacement of the control member by means of gas pressure until it abuts against the control member, while a circular-cylindrical outer surface of the sliding pin and a circular-cylindrical inner surface of the holder constitute a first gap-shaped leakage resistance connected to the chamber and a circular-cylindrical outer surface of the holder and a circular-cylindrical inner surface of the control member constitute a second gap-shaped leakage resistance connected to the first leakage resistance.
The invention further has for its object to provide a device for ultrasonically deforming an article made of synthetic material by means of a sonotrode which can be pressed against the article by a displacement device, the contact force between the sonotrode and the article being kept below an adjustable maximum value.
The device according to the invention is for this purpose characterized in that it is provided with a force limiter comprising a control member displaceable with respect to a holder and a chamber, whose shape is determined inter alia by the holder and the control member and which is located between a gas supply and a leakage resistance, the leakage resistance comprising a gap which is connected to the chamber, is located between at least one circular-cylindrical inner surface and at least one circular-cylindrical outer surface and is annular in sectional views transverse to the direction of displacement of the control member.
The invention further has for its object to provide a method of the kind mentioned in the opening paragraph, in which the contact force between the two articles is kept below an adjustable maximum value.
The method according to the invention is for this purpose characterized in that a relative displacement between a holder and a control member of a pneumatic force limiter is obtained by means of the contact force as soon as a force of pre-stress determined by gas pressure of the force limiter coupled to the first article is exceeded and the said displacement is ascertained by a detector which is arranged in the force limiter and supplies a stopping signal to the motor drive of the first article, the time required for stopping the motor drive being shorter than the time required for the relative displacement between the holder and the control member of the pneumatic force limiter.
A particular embodiment of the method, which yields a very regular and highly reproducible deforming process when synthetic materials are deformed ultrasonically, is further characterized in that, as soon as the stopping signal is supplied by the detector to the motor drive, the sonotrode subjected to ultrasonic vibrations is pressed into the synthetic material of the second article with a force which is determined inter alia by the force of pre-stress of the force limiter and the weight of the sonotrode and the control member, the motor drive being switched on again only when the detector ascertains that the relative displacement between the holder and the control member has fallen below a given threshold value due to the deformation of the second article.